Radio frequency (RF) signals typically experience significant losses during propagation from a transmitting antenna to a receiver antenna. Accordingly, when a receive signal is received from on the receiver antenna, the receive signal often needs to be amplified in a receiver so that the receiver signal is provided at an appropriate power level for processing. However, amplifying a signal can introduce noise and thus low-noise amplification (LNA) circuits may be employed to minimize the amount of noise introduced into the receive signal by amplification. LNA circuits amplify the receive signal while being tuned around a receive frequency band associated with the receive signal to reduce the amount of noise introduced during amplification.
Nevertheless, in today's world of mobile communication, mobile communication devices are regularly transported from location to location, thus varying the distance between the transmitting antenna and the receiving antenna. If the receive antenna is too close the transmitting antenna, the receive signal may not need to be amplified or the amplification by the LNA circuit may be at a power level that is too high. As a result, the receiver may be structured to have an RF circuit that includes the LNA circuit and a bypass path coupled to provide a bypass for the receive signal around the LNA circuit. In an amplification mode, the bypass path of the RF circuit is opened allowing the LNA circuit to amplify the receive signal. On the other hand, in a bypass mode, the bypass path of the RF circuit is closed and the receive bypasses the LNA circuit.
Unfortunately, while tuning the LNA circuit to the receive frequency band reduces noise during amplification, it introduces a maximum attenuation peak near the receive frequency band, which may introduce high insertion losses to the receiver signal in bypass mode. FIG. 1 illustrates the S21 response of a prior art RF circuit and includes a power gain frequency response 10 of a prior art LNA circuit during the amplification mode and the attenuation frequency response 12 of a prior art bypass path during the bypass mode. As shown in FIG. 1, the LNA circuit is tuned so that a power gain resonance frequency band 14 of the power gain frequency response 10 is well within a receive frequency band 16 of the receive signal. In fact, in this case a resonant frequency 18 of the power gain resonance frequency band 14 is provided so that, in the amplification mode, the maximum gain is provided in the middle of the receive band 16. However, the LNA circuit also loads the bypass path and this same tuning of the LNA circuit creates a notch 20 in the attenuation frequency response 12. This introduces excessive insertion losses in the bypass path. In other words, a maximum attenuation resonance frequency 22 of the notch 20 is too close to the receive frequency band 16 in the bypass mode and a notch band 24 includes the receive frequency band 16 of the receive signal.
Attempts have been made to solve this problem. FIG. 2 illustrates the S21 response of another prior art RF circuit having the power gain frequency response 10 of another prior art LNA circuit during the amplification mode and the attenuation frequency response 12 of another prior art bypass path during the bypass mode. In this case, the notch band 24 has been provided outside of the receive frequency band 16 and thus the bypass path provides lower insertion losses during the bypass mode. On the other hand, this was accomplished by tuning the LNA circuit so that the resonant frequency 18 of the power gain resonance frequency band 14 is further away from the receive frequency band 16. While the receive frequency band 16 is still within the power gain resonance frequency band 14, the receive frequency band 16 is closer to the edges of the power gain resonance frequency band 14. As a result, during the amplification mode, the LNA circuit amplifies the receive signal with less gain and may introduce greater amounts of noise into the receive signal.
What is needed is an RF circuit and methods of operating the same that provide a higher gain to amplify the receive signal during the amplification mode but do not introduce large insertion losses during the bypass mode.